The present invention relates generay to a single screw positive displacement mechanism capable of maintaining a proper mesh between its main rotor and mating gate rotor when there are few gate rotor teeth engaged in the main rotor and further providing for main rotor thread baffling between the main rotor chamber section and mechanism inlet. The invention also provides for the reduction of contact forces between chamber gate rotor sealing flanks thereby reducing gate rotor tooth wear.
Recent developments in shipboard operations require an efficient high-pressure compressor. Devices having a compressed air flow of approximately 3,000 psi are desirable. One device generally suitable for shipboard operations is a positive displacement type machine known as a single screw mechanism. Single screw mechanisms can be made to operate as a compressor, an expansion machine, a pump, a hydraulic motor, or the like.
The primary components of a single screw mechanism are a main rotor, a gate rotor, with or without gate rotor support, and a main rotor housing. Main rotors are typically provided with at least one thread and are driven and rotate about a central axis. The gate rotor, having at least one tooth in meshing engagement with the main rotor thread, is typically driven by the main motor. Often times the gate rotor is backed by a metal gate rotor support, which follows and supports each gate rotor tooth in the main rotor thread for purposes of reducing gate rotor tooth deflection due to operating loads. The main rotor housing is fitted in close proximity to the main rotor and to the crests of the main rotor teeth and is provided with at least one port leading to a suction, or inlet, plenum, and at least one additional port leading to a discharge plenum.
The general operation of a single screw mechanism is as follows: Gas is drawn into the main rotor thread from the suction plenum. When the thread is filled with gas, a gate rotor tooth rotates into position and, in cooperation with the main rotor casing, closes the thread to form a compression chamber. As the main rotor turns, the gate rotor tooth proceeds through the main rotor thread, reducing the compression chamber volume, and thereby compressing the gas. When the desired gas pressure is achieved, the edge of the rotating main rotor thread uncovers a discharge port in the main rotor casing and the compressed gas is expelled into the discharge plenum.
Often times high-pressure single screw compressors have internal leakage that reduces both the volumetric and isentropic efficiencies of the device. In order to reduce the internal leakage and thereby increase the compressor efficiency, it may be necessary to reduce tooth penetration into the main rotor. Reducing the tooth penetration reduces the gate rotor tooth flank lengths and thus the tooth flank leakages. One problem with reducing the gate rotor tooth penetration is that the number of ate rotor teeth engaged in the main rotor is reduced, resulting in timing and meshing problems between the main rotor and gate rotor. Additionally, high-pressure single-screw compressors can experience rapid gate rotor tooth flank wear on critical sealing surfaces on account of the heavy contact forces which exist between the gate rotor and main rotor at high operating pressures. This causes the gate rotor teeth flanks to wear resulting in less effective sealing against internal leakage. Volumetric and isentropic efficiencies for the machine suffer as internal leakage increases.
One way to reduce internal leakage is to baffle main rotor thread crests. More specifically, baffling is accomplished by sealing main rotor threads from the suction side of the device by placing additional main rotor thread crests or casing crests between the compression chamber portion of the mechanism and the mechanism inlet. These additional thread or casing crests reduce thread leakage and thereby improve the mechanism's efficiency.
As can be seen from the above discussion, from the standpoint of mechanism efficiency, it is desirable to reduce gate rotor tooth penetration into the main rotor in order to have less tooth flank leakage, which is essential for efficient high-pressure devices. It is also desirable to have a gate rotor configuration which resists wear in order to provide for reliable high-pressure operation, and it is further necessary at high pressures to baffle main rotor threads to reduce leakage and improve efficiency. Accordingly, for these and other reasons, the need exists for a device to insure the proper meshing of slightly penetrating gate rotor teeth with baffled main rotor threads while reducing gate rotor tooth wear at critical sealing surfaces.